Chromosome-specific DNAs will be prepared for all mouse and rat chromosomes by combining the technologies of chromosome microdissection and degenerate oligonucleotide primed PCR (DOP-PCR) amplification. Difficulties with flow sorting of chromosomes have prevented complete sets of chromosome-specific DNAs or libraries other than human from being made available. Microdissection will circumvent these difficulties and allow pure chromosome-specific DNAs to be prepared from all chromosomes. A single degenerate primer will allow amplification of DNA from both mouse and rat. Cot1 DNA for use as competitor in hybridizations will be prepared for both species. Chromosome-specific DNAs will be tested for specificity by painting. Selected DNAs will be tested for representation by hybridization to a battery of unique chromosome-specific probes. Chromosome-specific DNAs will be cloned using conventional techniques. Chromosome-specific DNAs, Cot1 DNA, and protocols for use will be made available to the research community.